Biologically degradable foamed material particles

ABSTRACT

In a process for preparing expandable polymer beads based on a mixture made from 
     A. a biodegradable aliphatic/aromatic copolyester and 
     B. a biodegradable naturally occurring polymer, 
     the mixture A+B is melted in an extruder, mixed with a volatile organic blowing agent, extruded and pelletized.

The invention relates to expandable polymer beads based on biodegradablepolymers.

It is known that certain naturally occurring materials, e.g. celluloseand starch, are biodegradable, i.e. are decomposed by microorganisms, inparticular during composting, to give low-molecular-weight substances.Some synthetic polymers, too, such as polyesters, are biodegradable.Although purely aliphatic polyesters have good biodegradablility, theirlow level of mechanical and thermal properties gives them poorsuitability for practical applications. In contrast, aromatic polyestershave good mechanical properties but are not biodegradable.

WO 96/07687 discloses that copolyesters which comprise firstly aliphaticdiols and secondly both aliphatic and aromatic dicarboxylic acids asmonomer units have good mechanical and thermal properties and are at thesame time biodegradable. This publication also mentions foamed moldingsbut nothing is said concerning their production.

WO 96/15173 likewise describes biodegradable copolyesters made fromaliphatic diols and from a mixture of adipic acid and terephthalic acid,and modified by incorporating other esterification components. Thesepolyesters can be used to make foams, by mixing a polymer melt withblowing agents and foam-extruding the mixture.

A disadvantage of the foam-extrusion process is that it can produce onlyfoam moldings of simple shape, e.g. sheets or profiles. It is known thatfoam moldings of any desired shape can be produced by foaming andsintering expandable or, respectively, expanded polymer particles, forexample those based on polystyrene or, respectively, on polyolefins.

Aliphatic/aromatic copolyesters degrade relatively slowly duringcomposting. The degradation process can be adjusted as desired byadmixing naturally occurring polymers which degrade rapidly.

EP-A 667 369 describes moldable foams made from biodegradable mixturesof thermoplastic starch and a synthetic thermoplastic polymer, e.g. analiphatic polyester. The polymer mixtures here are extruded togetherwith from 2 to 20% of water as blowing agent and pelletized to giveexpandable beads, which are then foamed in an extruder. The watercontent has to be set very precisely for each particular case. If thecontent is too high there is a risk that the polyester will hydrolyzeand that the starch will begin to dissolve. Complicated drying of thefoam beads is then required.

Finally, WO 97/44388 again describes foams made from moisture-resistantbiodegradable mixtures of naturally occurring polymers, e.g. starch,with hydroxy-functional polyesters. These biodegradable materials, too,were foamed by the foam-extrusion process, using water as blowing agent.

It is an object of the present invention to provide expandable beadsmade from mixtures of biodegradable naturally occurring polymers andbiodegradable polyesters, where there are none of the disadvantagesmentioned.

We have found that this object is achieved by melting a mixture madefrom

A. a biodegradable aliphatic/aromatic copolyester and

B. a biodegradable naturally occurring polymer

in an extruder, mixing with a volatile organic blowing agent, extrudingand pelletizing.

Polyesters suitable for the novel process are described in thepublications cited: WO 96/07687, WO 96/15173 and WO 97/44388.

Particularly preferred aliphatic/aromatic polyesters include polyestersin which the essential components present are:

a) an acid component made from

a1)from 30 to 70 mol % of at least one aliphatic or cycloaliphaticC₂-C₂₀ dicarboxylic acid or of ester-forming derivatives or mixtures ofthese

a2) from 30 to 70 mol % of at least one aromatic dicarboxylic acid or ofester-forming derivatives of these, and

b) a diol component made from at least one C₂-C₁₂ alkanediol or from aC₅-C₁₀ cycloalkanediol, and also, if desired,

c) a component selected from the group consisting of

c1) at least one compound having at least three groups capable of esterformation

c2) at least one isocyanate and

c3) at least one divinyl ether,

or mixtures of the same.

The incorporation of component c) results in long-chain branching.

Possible aliphatic and, respectively, cycloaliphatic acids and thecorresponding derivatives a1 are those mentioned above. Particularpreference is given to adipic acid or sebacic acid, ester-formingderivatives of each of these or mixtures of the same.

Aromatic dicarboxylic acids a2 are generally those having from 8 to 12carbon atoms, for example terephthalic acid, isophthalic acid,2,6-naphthoic acid and 1,5-naphthoic acid, and also ester-formingderivatives of the same. The di-C₁-C₆-alkyl esters, and also theanhydrides, are particularly suitable here. The aromatic dicarboxylicacids or ester-forming derivatives of the same a2 may be usedindividually or as a mixture of two or more of the same. Particularpreference is given to terephthalic acid or ester-forming derivatives ofthe same, for example dimethyl terephthalate.

The partly aromatic polyesters are characterized by a molar mass (Mn) offrom 5000 to 100,000 g/mol, in particular from 10,000 to 40,000 g/mol,with a viscosity number of from 50 to 400, in particular from 100 to 300ml/g (measured in o-dichlorobenene/phenol; weight ratio 50/50, at aconcentration of 0.5% by weight of polymer and at 25° C.).

Naturally occurring polymers B are preferably starch or cellulose orelse derivatives of the same, e.g. esters or ethers, or elsepoly(hydroxycarboxylic acids), such as polylactic acid orpolyhydroxybutyric acid, or else poly(aminocarboxylic acids), such aspolyaspartic acid or derivatives of the same.

The weight ratio A:B is from 9:1 to 1:9, preferably from 3:1 to 1:3 andin particular from 2:1 to 1:2.

The biodegradable polymers A+B may comprise additives, such asstabilizers, neutralizing agents, lubricants, release agents,antiblocking agents, nucleating agents, flame retardants, nonfluorescingdyes or fillers.

In the novel preparation process, the polymer mixture A+B is melted inan extruder, if desired together with the additives, preferably at from70 to 220° C., and mixed with a volatile blowing agent. Preferredblowing agents are C₃-C₈-hydrocarbons, halogenated hydrocarbons andC₁-C₃-alcohols. Particular preference is given to butane, pentane,isobutane and ethanol. The amount of the blowing agent used ispreferably from 1 to 20% by weight, based on A+B.

After discharge from the extruder, the melt extrudate may either becooled in a water bath and then pelletized or may be pelletized by hotcutting immediately after being discharged, followed by cooling of thepellets.

The resultant expandable polymer beads generally have a diameter of from0.2 to 5 mm. They may be expanded by conventional methods, e.g. usingsteam or hot air, to give foam beads.

The foam beads may be further foamed and sintered as usual in closedmolds using steam or hot air to give foam moldings of any desired shape.

The foam beads themselves may be used as a soil conditioner in fieldsand gardens, in which case the degradation time can be adjusted asdesired by way of the ratio A:B.

Foam moldings made from the biodegradable foam beads may be used assingle-use tableware, packaging trays, dishes or cups. Once comminutedthey can be composted and thus degraded.

We claim:
 1. A process for preparing expandable polymer beads based on amixture comprising A. a biodegradable saturated aliphatic/aromaticcopolyester and B. a biodegradable naturally occurring polymer in aweight ratio A:B of from 9:1 to 1:9, which comprises melting the mixturemade from A +B in an extruder, mixing the same with a volatile organicblowing agent, and extruding and pelletizing the same.
 2. A process asclaimed in claim 1, wherein the polyester A is a condensation productcomprising a. an acid component comprising a1. from 30 to 70 mol % of analiphatic or cycloaliphatic C₂-C₂₀ dicarboxylic acid or of ester-formingderivatives of these, a2. from 70 to 30 mol % of an aromaticdicarboxylic acid or of ester-forming derivatives of these, b. a diolcomponent made from a C₂-C₁₂ alkanediol or from a C₅-C₁₀cycloalkanediol, and also, if desired, c. other ester-formingcomponents.
 3. A process as claimed in claim 1, wherein the polymer B isstarch or cellulose or derivatives of the same.
 4. A process as claimedin claim 1, wherein the volatile organic blowing agent is a C₃-C₈hydrocarbon or a C₁-C₃ alcohol.
 5. A process as claimed in claim 1,wherein the amount of the blowing agent used is from 1 to 20% by weight,based on A+B.
 6. A process as claimed in claim 1, wherein the meltextrudate discharged from the extruder is cooled in a water bath and thesolidified extrudate is pelletized.
 7. A process as claimed in claim 1,wherein the melt extrudate discharged from the extruder is pelletizedimmediately after being discharged and the pellets are cooled.
 8. Amethod of foaming a biodegradable polymer foam comprising foaming theexpandable polymer beads as claimed in claim
 1. 9. The process asclaimed in claim 1, wherein the polymer B is an ester or an etherderivative of starch or cellulose.